In engine diagnostic apparatus it is quite common to employ a signal derived from the voltage pulses applied to the engine igniters to control the operation of testing apparatus. For example, one or more igniters may be selectively rendered ineffective and by employing a counter or some other timing device, it is possible to determine in accordance with the number of ignition pulses which cylinder would be firing at any particular time. It is also old to utilize such a signal for controlling the horizontal sweep of a cathode ray tube forming part of such an engine diagnostic apparatus. Regardless of the nature of the testing operation being performed, it is often desirable to have the testing operations start at a point prior to the occurrence of the ignition pulse being considered in the test. In the Marino U.S. Pat. No. 3,572,103 and the Marino and MacCrea U.S. Pat. No. 3,573,608, this was accomplished by deriving a signal across the distributor points and employing this signal to produce a square wave of the same frequency as the frequency of occurrence of the pulses produced by the opening and closing of the distributor points. This system worked quite well where the distributor points were readily accessible or where a connection to the primary winding at the point where it is connected to the distributor points produced a voltage accessible for use in the engine diagnostic apparatus. Where, however, inductive pickups are used or where, as is the case with electronic ignition, it is often very difficult to connect readily to any point in the system which corresponds to the connections across the distributor points, it is often necessary to connect to the secondary winding of the ignition coil. This produces a signal which indicates approximately the time that a voltage is applied to the igniter but which does not provide a ready indication of any intermediate point spaced from the ignition pulse by a predetermined portion of the cycle. Due to the wide variation in engine speed at which such testing operations must be performed, it is impossible to use the ignition pulse to generate a square wave of fixed duration and have this suitable for all engine speeds. It is hence desirable to have a voltage wave which, like that appearing across the distributor points, has a definite fixed intermediate point which occurs at a point in the cycle which is relatively constant percentagewise, regardless of the frequency of the occurrence of the ignition pulses.
Where the counter is employed, a further problem often arises. It is necessary to make sure that the counter starts its counting operation at a predetermined point in the engine cycle. Thus, it is quite common to employ the ignition signal being supplied to one particular engine igniter to generate a further pulse which in turn is used to make sure that the counter is reset at the time that an ignition signal would normally be supplied to that particular igniter. The problem that arises with this arrangement is that the connection to the ignition wire to the selected igniter may run close to other ignition wires so that a signal may be picked up from ignition wires leading to other igniters. This may result in the counter being falsely reset at the wrong time. While it has been proposed to provide an arrangement for preventing such a reset signal being supplied to the counter at a time other than that at which an ignition signal would normally be supplied to the selected plug, prior arrangements have had a disadvantage that if the diagnostic apparatus was connected to an engine which was running before the power supply for the apparatus was turned on, it would be possible to get into a situation in which the counter never was allowed to start its cycle.